A highly anticipated women’s race was billed as another battle between Hassan and Assefa – the gold and silver medallists in Paris last summer.
The pair were among the lead group early on but by the 20km (12.5-mile) mark, Hassan was five seconds back with Assefa and Jepkosgei maintaining a relentless pace.
It looked set for a close finish but Assefa, the former outright world record holder, kicked for the line and from being neck-and-neck with Jepkosgei at 35km (just under 22 miles), she was 56 seconds ahead at 40km and held treble that advantage by the line.
“I won second here last year so to win this year is very special. I am really, very happy,” Assefa told BBC One.
“Last year I did have some problems with the cold and my hamstring tightened up towards the end. This year the weather suited me better.
“I am really pleased with how the race went.”
The field for the men’s elite race was touted as the best ever in London and the lead group was still comprised of nine athletes after 30km (18.64 miles).
Sawe made his move just after the 31km (19.26 miles) mark and although the competition included Kiplimo, Mutiso, four-time winner Eliud Kipchoge and Olympic champion Tamirat Tola, none could match him.
The Kenyan stretched his lead and eventually crossed the line a minute and 10 seconds ahead of his nearest challengers.
“I am so happy. This is my fastest time for the London Marathon,” he told BBC One.
“I was well prepared for this race and that is why it is an honour for me to have won.”